110 GEOLOGY OF THE HIGH PLATEAUS. 



Geotjp III.— SUB-BASIC ROCKS— PROPYLITE AND ANDESITE— Continued. 



IV. BASALTS. The classification and subdivision of the basalts pre- 

 sent some difficulty. In the basic lavas we have occvirrences in which the 

 minerals leucite and nephelin replace wholly or in part the feldspars, and a 

 question arises as to the importance which is to be attached to this substitur 

 tion. In the other great groups the subdivisions have rested upon texture and 

 general habitus of the sub-groups as well as upon the occuiTence of accessory 

 and subordinate minerals in conspicuous quantity. In the acid and sub- 

 acid rocks accessory minerals are relatively in small proportions and varia- 

 tions of texture and habit very strongly pronounced. In the basic rocks 

 the reverse is .true — the accessory minerals are more numerous, almost 

 rivaling the primary ones, while the texture, though considerably vai'ied, 

 is far less so than in the acid rocks. These considerations would lead us 

 to rest the subdivisions rather upon a mineralogical basis than upon a tex- 

 tural one. Some authors separate doleritetroui the so-called "true basalts" 

 on textural grounds, the former being macrbscopically crystalline while the 

 basalts proper exhibit distinct crystals only under the microscope. Even 



